/Engine/Source/Runtime/Engine/Classes/Engine/RendererSettings.h
Enumerates available options for alpha channel through post processing. The renderer will always generate premultiplied RGBA with alpha as translucency (0 = fully opaque; 1 = fully translucent).
namespace EAlphaChannelMode { enum Type { Disabled = 0, LinearColorSpaceOnly = 1, AllowThroughTonemapper = 2, } }